The present disclosure relates to high intensity electric discharge lamps. It finds particular application in those instances where a high temperature lamp coating capable of transmitting visible light and filtering UV and microwave light is needed. However, it is to be appreciated that the present disclosure will have wide application throughout the lighting industry.
Lamps for which the present disclosure may prove suitable include any lamp characterized by the inclusion of a discharge envelope of quartz or ceramic containing a discharge-supporting filling of gas or vapor, for example. The lamp usually includes at least one pair of electrodes with a gap of at least 3 mm between which an electric discharge passes in operation of the lamp. An electric current is supplied to the electrodes from a source exterior to the lamp envelope via what is commonly called a ribbon seal. This seal generally comprises a strip of refractory metal foil, commonly of molybdenum, having one end thereof electrically connected to a respective electrode, and the opposite end in electrical contact with a refractory metal rod which passes through the end wall of the envelope to provide an external lead. The foil, electrodes, and lead rods are embedded in the fused silica envelope wall.
In some lamps, the quartz envelope is doped with cerium, which absorbs light in the ultraviolet (UV) wavelength range. However, use of this type of material suffers from several drawbacks. One such drawback is that doping of the internal lamp surface of the quartz envelope with cerium lowers the anneal point. Quartz generally maintains its integrity up to an operating temperature of about 1250° C., above which temperature the quartz begins to soften and eventually causes lamp failure. Doping the quartz with cerium, which absorbs unwanted UV emissions, unfortunately lowers the anneal point of the quartz by up to 100° C., thus making it operationally functional only to 1150° C. Ceramic envelopes, which are not susceptible to cerium doping, suffer from a lack of a suitable UV barrier.
Another drawback to using cerium doped quartz is that the inner lamp surface does not efficiently reflect the UV emissions, but instead absorbs this energy. As noted, the energy eventually causes the quartz to soften and fail. An additional problem is that the UV energy is not redirected for further use by such envelope compositions. Instead, the energy is wasted.
It is known to use internal lamp envelope coatings to address these issues. However, a continuing problem remains with regard to coatings suitable for high temperature application, which do not suffer from the noted drawbacks, including lowering of the anneal point which results in early lamp failure and wasted lamp energy.
The invention disclosed herein is intended to provide a lamp coating suitable for use at high temperatures. The coating contemplated herein, due to its composition, does not interact or react with quartz or ceramic envelope material, thus the envelope maintains its integrity, increasing overall lamp life and efficiency. The coating further efficiently reflects UV light back into the arc so that the energy from this light wavelength is not wasted. Similarly, unwanted microwave radiation is reflected back into the lamp interior as opposed to being transmitted. Use of the reflective coating herein results in improved thermal and electrical performance of the lamp.